1. Field of the Invention
The present invention relates to, in general, electrocatalysts for fuel cells using support bodies resistant to carbon monoxide poisoning. More specifically, the present invention is directed to an electrocatalyst for fuel cells comprising activated metal particles acting as a catalyst and a support body for adsorbing such catalyst particles, characterized by using a porous carbon material as a support body, instead of conventionally used carbon black, in which such a carbon support body can be further adsorbed with an activated metal and has pores penetrating into an interior thereof with a diameter smaller than a kinetic diameter of carbon monoxide, whereby carbon monoxide inevitably produced upon reforming hydrocarbon fuels is prevented from coming into contact with the activated metal adsorbed to the inner surface of each pore of the support body, and deactivation of the activated metal due to adsorption of carbon monoxide is restrained, therefore resulting in preventing degradation of fuel cell performance by carbon monoxide.
2. Description of the Prior Art
In general, fuel cells function to directly convert chemical energy produced by oxidation of fuels to electrical energy. Such a fuel cell has the same function as typical chemical cells in view of using oxidation and reduction reactions. However, different from chemical cells using cell reaction in a closed system, the fuel cell acts as a certain electric power generator while a reactant is continuously fed from the outside and a reaction product is continuously discharged to the outside.
As for the fuel cell, hydrogen in hydrocarbon fuels, such as methanol or natural gas, is electrochemically reacted with oxygen in air to immediately generate electrical energy. That is, the fuel cell serves as a clean electric power generator having high efficiency. Since development of fuel cells for use in spacecraft or for military purposes in USA in the 1970s, intensive research into such fuel cells for use as general power sources has been ongoing. At present, research and development of practical fuel cells have been actively carried out in developed countries, such as USA, Japan and Europe.
Meanwhile, the fuel cell is classified into alkaline-, phosphoric acid-, molten carbonate-, solid oxide- and polymer electrolyte membrane-types, depending on the used electrolytes. Further, the cell having operating temperatures of 300° C. or less is referred to as a low-temperature type, while the cell having operating temperatures of 300° C. or more is referred to as a high-temperature type.
Reaction of hydrogen and oxygen, which are fuel of the fuel cell, at an electrode leads to production of water and electricity. As such, oxygen contained in air may be used as it is, whereas hydrogen may be used after being extracted from a separate storage container containing hydrogen in an amount necessary for the fuel cell.
However, since hydrogen is difficult and dangerous to handle because of its volatility, hydrocarbon fuels including methanol, LNG, gasoline, etc., are reformed to hydrogen, which is generally used for the fuel cell.
When hydrogen reformed from the hydrocarbon fuel is used, it is advantageous in terms of stability, storability and convenience, compared to direct use of hydrogen as a fuel, but has the following drawbacks.
In the course of reforming the hydrocarbon fuel, carbon monoxide and carbon dioxide are inevitably generated along with hydrogen. Such carbon monoxide is strongly adsorbed to a surface of platinum used as a catalyst of the electrochemical reaction, thus drastically degrading the fuel cell's performance.
Therefore, in order to efficiently use the hydrocarbon fuel which is relatively easy to handle, instead of directly using hydrogen which is difficult to handle, degradation of the fuel cell performance by carbon monoxide, which is produced with hydrogen upon reforming the hydrocarbon fuel, should be restrained as much as possible. For this, there are proposed methods of alloying the platinum catalyst with a second metal.
FIG. 1 shows a conventional catalyst structure. As shown in FIG. 1, carbon black is used as a support body 12, on which small particles of an activated metal 11 are bonded as a catalyst. Such a catalyst structure is completely exposed to a fuel gas. Hence, with the intention of decreasing adsorption energy between carbon monoxide and the surface of activated metal exposed to the fuel gas, platinum as the activated metal is not used alone but used in an alloyed state.
The second metal, which is alloyed to platinum to decrease adsorption energy with carbon monoxide, can be one of various types of metals. As a commercially available catalyst, there is widely used a Pt—Ru/C catalyst using ruthenium as the second alloying metal.
The ruthenium-added alloy catalyst has resistance to carbon monoxide poisoning according to the following Chemical Reaction 1:Ru+H2O=Ru—OH+H++e−Pt—CO+Ru—OH=CO2+H++e−+Pt+Ru  Chemical Reaction 1
According to the above chemical reaction, two methods for preventing adsorption of carbon monoxide to the platinum catalyst are employed. That is, one of the two is to decrease bonding energy between platinum and carbon monoxide, and the other is to effectively oxidize carbon monoxide.
However, in spite of the above two effects, degradation of the fuel cell performance by carbon monoxide cannot be completely prevented yet. Only when concentration of carbon monoxide is 10 ppm or less, fuel cell performance is satisfactorily maintained. As the concentration of carbon monoxide gradually increases over 10 ppm, the fuel cell performance is degraded. In particular, at 100 ppm carbon monoxide, the fuel cell performance is considerably degraded.
As mentioned above, hydrogen gas produced by reforming the hydrocarbon fuel contains essentially carbon monoxide, carbon dioxide, water and unreacted fuel. Of these components, carbon monoxide is the most problematic component. Therefore, in order to prevent degradation of the electrode performance by such carbon monoxide, a variety of methods have been proposed. Such methods are largely divided into two kinds as described below.
First, there is employed a method of using a metal alloy. As an alloyed platinum catalyst, use is made of PtRu, PtSn, PtOs, PtRh, PtIr, PtPd, PtV, PtCr, PtCo, PtNi, PtFe, PtMn, PtCoMo, PtWO3, PtCoWO3, etc. However, recently reported studies disclose no alloying methods capable of completely preventing degradation of the cell performance by carbon monoxide.
According to recent literature, in the presence of 100 ppm carbon monoxide in the reformed gas, the fuel cell shows a voltage drop of 150-200 mV at a certain current density (200 mA/cm2). Further, upon use of commercially available catalysts from Johnson-Matthey, E-TEK and Tanaka Co. Ltd., the voltage drop of fuel cells is reported to be similar to the 150-200 mV as stated above, or worse.
Second, there is employed an air-bleeding method, with the intention of solving poisoning problems of the platinum catalyst by carbon monoxide. Carbon monoxide adsorbed to the platinum catalyst is eliminated while air as an external oxygen source is supplied slowly. As such, when a reformed gas and air are slowly supplied to a certain electrode showing performance degradation, such an electrode is recovered.
However, the above method is effective only for a short-term period. In addition, since the reaction per se is highly exothermic, a platinum metal is agglomerated and does not function as a catalyst. In the case of the polymer electrolyte membrane fuel cell, a polymer membrane used as an electrolyte is easily punctured, thus decreasing durability of the cell.